Scientia

Barbara McClintock was years away from her colleagues

February 27, 2012 | Author: Freelance Writer Steven A. Edwards, Ph.D.

Barbara McClintock (1902-1992) was a geneticist whose discoveries were increadibly advanced for her time. (Photo: Courtesy of the Barbara McClintock Papers, American Philosophical Society and the National Library of Medicine)

At a time when molecular biologists struggled to devise a stable biochemical model for genes, Barbara McClintock discovered genetic transposition, or genes that are able to change position within the genome. However, she did not receive recognition for her work until late in her career.

McClintock was a cytogeneticist,
best known for her discovery in 1948 of transposons, mobile genetic elements
that do not necessarily stay fixed on the chromosome, but she had made many
fundamental discoveries before that.
She invented a method for visualizing the chromosomes of maize and used
it to visualize the crossing over of chromosomes during meiosis for the first
time. She was the first to
describe ring chromosomes formed after X-ray irradiation, and demonstrated the
necessity for the nucleolus organizer on maize chromosome six. Her work on X-ray generated mutations
prepared her for the unusual properties of the transposons. These she called ‘controlling
elements’, because she believed that they were important for gene
regulation.

McClintock gave a famous
seminar on transposons at a Cold Spring Harbor Symposium in 1951, which was not
at all well-received. Her mostly
male colleagues were dubious if not outright hostile to her ideas. She ceased publishing on transposons in
1953, when she realized “acutely, the extent of disinterest and lack of
confidence in the conclusions” she was drawing.

McClintock
is often made the poster woman for gender discrimination in the sciences, but
this characterization is really not fair.
It is true that early in her career she complained of her career
prospects while an assistant professor at the University of Missouri. But in 1944, at the age of 44,
McClintock was made a member of the National Academy of Sciences, only the
third woman to be so honored. A
year later, she was elected the first woman president of
the Genetics Society of America.

Not
until 1983 did she receive a belated Nobel Prize for her work on
transposons. But the delay in
recognition was less about her gender, than the fact that she was way ahead of
the molecular biologists of her day.
They were having enough trouble coming up with a biochemical model for a
genome that was stable from generation to generation without having to
accommodate genetic elements that clearly were not. “One must wait for the right time for conceptual change,”
wrote McClintock.

Today,
it is clear that much of what was once considered ‘junk DNA’ in the human
genome is composed of transposons, some virally-related, which may represent
fast-forward mechanisms for evolutionary change. For instance, the P elements of Drosophila were not present
in the laboratory strains isolated in 1905 but are now ubiquitous in wild type
flies. The mariner-like elements
have managed an impressive horizontal spread through hundreds of species of insects,
vertebrates and even plants.
The positive aspects of such genetic parasitism are hard to ascertain,
but the suspicion is that there must be some; otherwise it would not be so
prevalent.